The present invention relates to a method for monitoring of the dry line and for control based upon the dry line in a Fourdrinier paper machine. The invention also relates to an apparatus for carrying out said method.
Essential part of the Fourdrinier paper machine is the plane wire on which the dilute wood fibre pulp is fed and on which it settles forming a web. The web formation process essentially determines the quality of final product, since a major part of the water in pulp is removed through the wire, and the position of fibres with regard to each other does not change any more in the dryer part following the wire. The most important actuators which affect web formation and through it the quality of the paper or board are located in advance of the wire or in its immediate neighbourhood.
In order to reach a final product of even quality, it is important that the properties of the pulp web are measured as early as possible, i.e. already at the wet end of the paper machine. By means of stated actuators one may then reach a fast control and avoid the delay which is characteristic to conventional control based on measurements carried out at the dry end. However, practical methods for direct measurement of the pulp web at the wet end have been almost completely missing until recent times. The invention to be described lower down presents a new method for measurement at the wet end and control based on it.
The method of measurement to be presented is directed to the dry line which is related to the disappearance of water from the surface of the pulp web and is found at the location were the water or liquid (fluid) which behaves like water vanishes from said surface. The part of the web which precedes this location can be found glossy or specularly reflecting, due to the light reflected at places by it, while such a gloss cannot be observed on the part following the dry line.
In an industrial paper machine, the dry line is irregular in the cross direction and at the same time variable also in the machine direction. The gloss of the water surface found at an inspection of the wire is not uniform, but consists of spots which being brighter than their environment transmit light reflecting it from various sources of light, like from lamps of the factory hall, to eyes of the observer. A spot corresponding to even a single source of light is then indefinite and scattered, because it is not the simple mirror image of that light source which is observable to the eye, but a nonuniform, glossy area which is limited for its size and has an indefinite boundary line, because the water surface of pulp above the moving wire and pulp layer is not very even and because its local inclination is variable. The glossy area on the web surface sometimes extends sometimes not to the dry line and the water surface of pulp forms narrow, long peaks whose observation is made particularly difficult by the unevenness of the gloss.
Despite of the deficient observing of the dry line, the machine operator relies regularly on his observations on it in some of his control actions, i.e. in local or remote adjustment of manually controllable actuators, also in the case of a paper machine which has been provided with automatic controls based on quantities measured at dry end or dryer part. In order to obtain a picture of the dry line in its totality, he has then to inspect the wire from different directions in order to observe such, reflecting spots which are limited By the dry line at its different locations. According to his subjective observations he concludes the deviation of the dry line from the wanted location, both for the average value and in order to adjust the feed flow at different locations of the cross direction.
In order to observe the dry line instrumentally, one has in some cases inspected the wire by means of a single, photoelectric detector or an optoelectric camera which may be conventional television camera or a camera based on semiconductor detector which consists of discrete elements being used in the manner which appears from the British patent No 1430420 or corresponding U.S. Pat. No. 4,831,641. In these connections one has also stated the possibility of using electromagnetic radiation which is outside the wavelength range of visible light, analogously with the use of visible light, for observation of physical object. The use of these detectors as such does not, however, result in a clear and correct image of dry line, nor in correct values of quantities which characterize it. This is due to reasons which are already known from the visual observation of the man and which mislead an instrumental observation. If the number or power of light sources is increased, difficulties are by no means decreased. On the contrary, the numbers and contrasts of separate glossy areas and levels of brightness grow up and the blinding increases which further hampers instrumental observation. In addition to this, the determination of the dry line by computer from an indefinite camera signal requires a complicated computer program and results in computations which demand much time, if it can be carried out at all with an accuracy required by the control of the web.
The described difficulties do not not appear in the method according to the Finnish patent No 75887 or the corresponding U.S. Pat. No. 5,011,573. In this method the wire is illuminated in such an angle that direct reflections from the surface preceding the dry line are not brought up and direct reflection of other light sources is also prevented. Under such conditions, the part following the dry line which reflects diffusely the light it receives, is observed, due to this light which it emits, as a brighter part of the pulp web than the part preceding the dry line.
The last method above detects the dry line in industrial use continuously with a good accuracy, as a data set which is renewed repeatedly. The change of power of illumination in the cross direction of the wire which is present in the method does not essentially hamper the use of the method, but may require the illuminators to be located a longer way out from the wire in order to reach a more even profile of illumination. In such a case, the need of illuminative and therefore electric power increases, especially if a dark pulp is observed which after the dry line absorbs a considerable part of the light. In some cases the structures or auxiliary devices of the paper machine may, for their part, prevent a practical installation or maintenance of the equipment implied by the method, if the components to be maintained are located e.g. above the wire. The new method to be disclosed in the following produces a more even and at same time not blinding illumination and this way a detection of the dry line and control of the web at an essentially lower electric power than the method presented by the patent. It can also be carried out in many such paper machine environments to which the patented method does not apply for structural reasons.